The present invention is directed to an apparatus and method for monitoring chain pull or tension, and in particular, to the use of a load cell-type force sensor positioned between components of a chain drive for tension monitoring.
Conveyor systems using chains as the driving mechanism and chain drives to give the conveyor motion and control conveyor speed are well known in the art. One example of these types of conveyor systems is a power and free system. Power and free systems are generally made up of a power track, a free track, and trolleys capable of travelling along the free track, the trolleys supporting one or more carriers. Each carrier then supports a load or article being conveyed. The trolleys are usually divided into leading and trailing trolleys. Each leading trolley in a power and free system includes a driving dog portion which extends towards the power track and which is engageable by a pusher dog carried by a moving chain on the power track. When the pusher dog and the driving dog are engaged, the leading or drive trolleys push along the free track by the moving power chain. When the driving dog is retracted, or otherwise disengaged from the pusher dog, the trolley stops moving, thus halting the carrier.
To move the power chain, one or more conveyor drives are utilized. Two typical types include caterpillar and sprocket versions. A sprocket drive delivers motion to the conveyor chain directly from the output side of a reducer through a sprocket whose teeth mesh with the lengths of the conveyor chain. A caterpillar drive transmits its driving force to the conveyor by means of a caterpillar chain made of precision steel rollers with driving dogs that mesh with the lengths of the conveyor chain. Depending on the size of the conveyor system, the drives can provide chain pulls of up to 12,000 pounds.
Caterpillar drives come generally in junior or standard categories. The smaller drives can be designed with either a fixed frame or a floating frame. Larger drives generally use floating frames. Caterpillar drives are usually installed along any horizontal straight run of a conveyor track.
Standard floating drives can be either a linear type or a rotary type. The linear type is generally built with an inner floating frame that is guided and supported by ball bearing wheels attached to an outer fixed frame. In contrast, a rotary drive is mounted on an inner floating frame that pivots around a reducer output shaft, the floating frame acting as a torque arm against the fixed outer frame. One or more compact coil springs counterbalance the normal chain pull and control the movement of the floating frame.
One example of a conveyor drive is disclosed in U.S. Pat. No. 4,222,481 to Dehne et al., hereby incorporated in its entirety by reference. With particular reference to FIGS. 7 and 8 of this patent, pivotal movement of the floating frame is easily opposed by a compression spring. The compression spring is arranged between a plate attached to the moveable frame and another plate secured to the fixed frame. The force of the compression spring biases the movable frame against the torque caused by drive pull. The Dehne et al. patent also discloses a shock absorber to further restrain pivotal movement of the frame. The shock absorber is mounted in a similar fashion as the compression spring, the absorber being arranged between the floating frame and the fixed frame.
The Dehne et al. patent also teaches that a limit switch can be provided to provide overload protection in case of excessive pivotal movement of the floating frame, such caused by a chain jam or the like.
Another prior art conveyor drive is disclosed in FIG. 1 and designated by the reference numeral 10., The drive is depicted in the same view as FIG. 4 of the Dehne et al. patent. Shown is a reducer 1, a reducer shaft 3, a bearing 5, and a drive sprocket 7. FIG. 1 does not show the caterpillar chain around the drive sprocket 7.
FIG. 1 also shows an I-beam 9 which provides support for the trolley of a power and free system. The driven chain of the power and free system travels in a direction perpendicular to the view shown in FIG. 1 and in a direction from the motor (not shown) towards the reducer 1.
The reducer 1 is also shown with an input shaft 11 and pulley 13. The reducer 1 is connected to the motor via components 11 and 13 in a conventional fashion.
The speed reducer 1 is supported by a floating frame 15, similar to the manner of support described in the Dehne et al. patent. The FIG. 1 embodiment uses a torque arm assembly 17 and a compression screw 19 to monitor the chain pull on the conveyor drive. The torque arm assembly 17 includes a rod 18, a compression spring 21 and a rod plate 25. One end of the rod 18 is attached to the fixed frame 27. The other end of the rod 18 is secured to the plate 25. The spring 21 is interposed between the plate 25 and a bracket 23 attached to the floating frame 15. In this configuration, the spring 21 biases the floating frame 15 in the direction A, in opposition to the drive torque occurring in the direction B.
The rod 18 has a strain gauge sensor 29 as a part thereof, the strain gauge sensor 29 monitoring the chain pull during conveyor operation. More particularly, the strain gauge sensor 29 is zeroed when the conveyor drive is at rest, i.e. zero chain pull. When the conveyor drive is operating and a drive torque B is applied to the floating frame 15, the amount of chain pull is monitored for conveyor operation control.
Another embodiment similar to FIG. 1 uses the torque arm assembly 17 without the strain gauge sensor 29, thereby relying on other methods and techniques to monitor chain pull. One such technique uses a strain gauge link as part of the conveyor chain itself. The gauge on the link is calibrated so that the output of the gauge corresponds to the tension in the conveyor chain. The conveyor chain is stopped and the link is installed in the chain, the link then travelling around the conveyor system. Output data can be collected either by an umbilical cord and a data recorder or can be stored in computer memory for later download. Although the data collected in this fashion can be analyzed to find potential problem spots in the system, this method does not allow for the collection of data at one point in the conveyor system over a long period of time.
There are advantages to looking at the pull exerted by the drive over a period of time such as with the FIG. 1 embodiment. One of the problems with this embodiment is that the strain gauge is incorporated into the drive frame. If the drive frame is redesigned for a new installation or use, the strain gauge must also be redesigned to fit the requirements of that particular drive. Alternatively, if the strain gauge is being added as a retrofit, an expensive and time consuming process is required to remove the torque arm and replace it with a strain gauge-containing torque arm assembly.
Consequently, a need has developed to provide simplified and improved methods and apparatus to monitor the chain pull in conveyor systems. The present invention solves this need by providing a simple but effective method to install a strain gauge on an existing conveyor as well as an improved conveyor drive apparatus. With the invention, the conveyor does not have to be stopped to install the strain gauge and must be stopped for just a short period of time for calibration.
Accordingly, it is a first object of the present invention to provide an improved chain tension monitoring apparatus.
Another object of the present invention is an apparatus for monitoring chain tension which is easily retrofitted on existing units.
A still further object of the present invention is a method and apparatus of monitoring chain tension using a force sensor that easily interfaces with a floating frame conveyor drive.
One other object of the present invention is a method and apparatus for monitoring chain pull which is easily installed and calibrated.
Other objects and advantages of the present invention will become apparent as a description thereof proceeds.
In satisfaction of the foregoing objects and advantages, the present invention provides an improvement in conveyor drives utilizing fixed and floating frames. In these types of conveyor drives, the floating frame supports a conveyor drive unit and the frame is biased against the fixed frame by a compression spring to oppose forces generated by the conveyor drive operation.
In accordance with the invention, a force sensor is arranged on a portion of the floating frame and is configured with respect to the compression spring to sense a maximum compression force when chain pull of the chain drive is zero. The floating frame can be a rotary type, a linear type, or other known conveyor drives utilizing floating frames. Preferably, the force sensor is a load cell sensor and the portion of the floating frame where the force sensor is located opposes a portion of the fixed frame.
In one embodiment, a clamp assembly can be used to assist in mounting the force sensor to the floating frame. In this embodiment, the clamp assembly can utilize a clamp plate which fixes the force sensor between the clamp plate and the floating frame. In this arrangement, the force of the floating frame against the force sensor is resisted by the fixed frame through the clamp plate.
The invention also includes a method of monitoring chain pull in a conveyor system that employs a conveyor drive using a fixed frame and a floating frame. The floating frame supports a conveyor drive unit, the floating frame biased against the fixed frame by one or more compression springs to oppose forces generated by drive operation. According to the inventive method, the force sensor contacts a portion of the floating frame. The force sensor is pre-compressed to a preset force level and the chain drive is operated to generate a given chain pull. Using the force sensor, the operating chain pull is measured for monitoring purposes.
The floating frame can either pivot about an axis thereof or move along its longitudinal axis. As part of the measuring step, the data generated by the force sensor can be stored in electronic form.
In one mode, the force sensor is arranged by first compressing the compression spring to create a space between the floating frame and the fixed frame. The force sensor is then inserted in the space and the compression spring is allowed to expand to fix the force sensor in place. During the insertion step, the conveyor drive can remain operating and only has to be stopped to measure a pre-compression load on the force sensor for ultimately calculating the chain pull when the chain drive is in operation.